The invention relates to the control of electrochromic devices, more particularly, the invention relates to a method and apparatus suitable for use in controlling a charge level of an electrochromic device.
The optical properties of electrochromic materials change in response to electrically driven changes in oxidation state. Thus, when an applied voltage from an external power supply causes reduction or oxidation of an electrochromic material, its transmittance properties change. In order to maintain charge neutrality, a charge balancing flow of ions in the electrochromic device is needed. By enabling the required electron and ion flows to occur, an electrochromic device utilizes reversible oxidation and reduction reactions to achieve optical switching.
Conventional electrochromic devices comprise at least one thin film of a persistent electrochromic material, i.e., a material which, in response to application of an electric field of given polarity, changes from a high-transmittance, non-absorbing state to a low-transmittance, absorbing or reflecting state. Since the degree of optical modulation is directly proportional to the charge transfer induced by the applied voltage, electrochromic devices demonstrate light transmission tunability between high-transmittance and low-transmittance states. In addition, these devices exhibit long-term retention of a chosen optical state, requiring no power consumption to maintain that optical state. Optical switching occurs when an electric field of reversed polarity is applied.
To facilitate the aforementioned ion and electron flows, an electrochromic film which is both an ionic and electronic conductor is in direct physical contact with an ion-conducting material layer. The ion-conducting material may be inorganic or organic, solid, liquid or gel, and is preferably an organic polymer. The electrochromic film(s) and ion-conductive material are disposed between two electrodes, forming a laminated cell.
When the transparent conductive electrode, adjacent to the electrochromic film, is the cathode, application of an electric current causes darkening of the film. Reversing the polarity causes electrochromic switching, and the film reverts to its high transmittance state. Typically, an electrochromic film such as tungsten oxide is deposited on a substrate coated with a transparent conductive film such as tin oxide or indium tin oxide to form one electrode.
Since an electrochromic device may be modeled as a non-linear passive device having an impedance dominated by a capacitive component, the amount of charge transferred to an electrochromic device is typically controlled by potential sources or current sources and current sinks.
In a known arrangement for controlling an EC device, an up/down counter is responsive to an up/down signal and a clock signal to produce a digital word representative of a desired EC charge level. Control logic is used to convert the digital word to a current source/sink programming signal suitable for causing a current source (or sink) to impart the desired charge level to the EC device.
Unfortunately, the above arrangement utilizes various components (e.g., current source and current sink transistors) having characteristics that tend to drift over time and temperature, thereby imparting more or less charge to the EC device than is otherwise indicated by the digital word produced by the up/down counter. In addition, EC devices themselves are subject to operational degradation over time and temperature. Moreover, the amount of energy required to charge an EC device is typically greater than the amount of energy required to discharge such a device. Thus, over a given period of time or temperature, an EC charge error may be accumulated such that the EC device may be significantly lighter or darker than desired.
A paper by J. P. Matthews et al., xe2x80x9cEffect of Temperature on Electrochromic Device Switching Voltages,xe2x80x9d Electrochimica Acta 44 (1999), discloses that switching voltages needed to color electrochromic devices vary with temperature. However, the paper does not disclose or suggest a method or apparatus for maintaining the charge delivered to an electrochromic device at a predetermined level.
The instant invention is directed to a method for delivering a substantially constant, predetermined charge to an electrochromic device, said method having a voltage compensation or adjustment requirement feature relative to varying ambient temperatures, and to an apparatus for use in an electrochromic (EC) control system in which components causing the charging and discharging of an electrochromic device are subject to drift errors and other errors.
The invention controls a charge/discharge voltage (or current) profile applied to an EC device to ensure that an appropriate voltage drop across the EC device is limited and/or maintained during charge and/or discharge modes of operation. The appropriate voltage drop is determined with respect to a temperature measurement proximate (i.e., near, on or within) the EC device. since the charge/discharge rate is defined by the voltage drop, a factor in the selection of an appropriate voltage is the appropriate charge/discharge rate of the device being controlled. The charge level of the device is monitored using a coulomb counter circuit having a topology designed to minimize interference in the operation of the EC device.
The invention simultaneously controls the total charge applied to an EC device and the rate at which that charge is applied to the EC device over a functional temperature range to control the EC device within a stable electrochemical limit to provide a useful lifecycle durability. A maximum rate of charge transfer is selected to avoid secondary electrochemical reactions of the controlled EC device. In one embodiment, a minimum rate of charge transfer may be provided to ensure that a minimum desirable rate of operation of the controlled EC device is maintained.
Specifically, the instant invention is directed to a method for controlling the rate of charge delivered to, or removed from, an electrochromic device, while maintaining the charge delivered to, or removed from, the electrochromic device at a predetermined or programmed level, where each of a plurality of levels corresponds to respective bleached or colored states, as the temperature proximate (i.e., near, on or within) the device varies, the method comprising the steps of: (a) sensing the temperature proximate the device; and (b) adjusting the voltage or current applied to the device based on the temperature sensed in step (a).